Working machine

ABSTRACT

A working machine includes: a machine body provided thereon with a traveling pump and a traveling motor; a traveling change-over valve shiftable between a first state where a rotation speed of the traveling motor is set to a first speed and a second state where the rotation speed is set to a second speed; an actuation valve operable to control a flow of operation fluid to an operation valve that changes a pressure of operation fluid to be applied to the traveling pump according to operation of an operation member; and a controller to perform a shock-mitigation control for reducing an opening degree of the actuation valve when the traveling change-over valve is shifted from the first state to the second state. The controller determines a reduction amount of the opening degree of the actuation valve reduced by the shock-mitigation control, based on a straight-traveling degree of the machine body.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Divisional of U.S. application Ser. No.17/394,612 filed Aug. 5, 2021, which claims priority to JapaneseApplication No. 2020-137193, filed Aug. 15, 2020, and JapaneseApplication No. 2020-137192, filed Aug. 15, 2020. The disclosures ofeach of the applications listed above are incorporated by referenceherein in their entireties.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a working machine such as a skid steerloader, a compact track loader, or a backhoe.

2. Description of the Related Art

A technique for reducing and increasing a speed of a working machine isdisclosed in Japanese Unexamined Patent Publication No. 2017-179922. Ahydraulic system for a working machine disclosed in Japanese UnexaminedPatent Publication No. 2017-179922 includes a hydraulic pump configuredto deliver hydraulic fluid, a hydraulic change-over valve configured tobe switched to any one of a plurality of switching positions accordingto a pressure of the hydraulic fluid, and a traveling hydraulic systemconfigured to change a speed according to the switching position of thehydraulic change-over valve.

SUMMARY OF THE INVENTION

In the working machine disclosed in Japanese Unexamined PatentPublication No. 2017-179922, a bleed fluid line is provided to apressure-receiving portion of the hydraulic change-over valve, whichmakes it possible to reduce a shift shock in accelerating ordecelerating the working machine. However, in the Japanese UnexaminedPatent Publication No. 2017-179922, a bleed fluid line has to beprovided to reduce the shift shock, which increases the number of parts.

In view of the problems of the conventional technique described above,the present invention intends to provide a working machine capable ofeasily reducing a shift shock.

In an aspect, a working machine includes a prime mover, a traveling pumpdriven by power of the prime mover so as to deliver operation fluid, atraveling motor configured to be rotated by the operation fluiddelivered from the traveling pump, the traveling motor having a rotationspeed shiftable between a first speed and a second speed that is higherthan the first speed, a machine body provided thereon with the primemover, the traveling pump and the traveling motor, a travelingchange-over valve shiftable between a first state where the rotationspeed of the traveling motor is set to the first speed and a secondstate where the rotation speed of the traveling motor is set to thesecond speed, a change-over switch operable to issue a speed-shiftinstruction selected between an acceleration instruction to shift therotation speed of the traveling motor from the first speed to the secondspeed and a deceleration instruction to shift the rotation speed of thetraveling motor from the second speed to the first speed, a travelingoperation device including an operation valve configured to change apressure of operation fluid to be applied to the traveling pumpaccording to operation of an operation member, an actuation valveprovided upstream or downstream of the operation valve to be fluidlyconnected to the operation valve so that the actuation valve is operableto control a flow of operation fluid to the operation valve, and acontroller configured or programmed to perform a shock-mitigationcontrol for reducing an opening degree of the actuation valve byoutputting a control signal to the actuation valve when the travelingchange-over valve is shifted from the first state to the second state.The controller is configured or programmed to determine a reductionamount of the opening degree of the actuation valve reduced by theshock-mitigation control, based on a straight-traveling degree of themachine body.

The actuation valve is configured so that the opening degree thereofbecomes larger as a control value corresponding to the control signalbecomes larger, and the opening degree thereof becomes smaller as thecontrol value becomes smaller. The controller is configured or programedto determine a reduction amount of the control value as the reductionamount of the opening degree of the actuation valve based on thestraight-traveling degree of the machine body, and to calculate amitigation value for the shock-mitigation control based on the reductionamount.

When a period for the shock mitigation control until the control valuereduced by the shock mitigation control reaches the mitigation value isdefined as a mitigation period, the controller is configured orprogrammed to reduce the control value at a first reduction rate keptconstant for the mitigation period from a start point thereof to an endpoint thereof.

When a period for the shock mitigation control until the control valuereduced by the shock mitigation control reaches the mitigation value isdefined as a mitigation period, the controller is configured orprogrammed to reduce the control value at a second reduction rate for afirst part of the mitigation period from a start point thereof to anintermediate point thereof, and to reduce the control value at a thirdreduction rate for a second part of the mitigation period from theintermediate point thereof to an end point thereof, the second reductionrate being greater than the third reduction rate.

The controller is configured or programmed to change a timing forshifting the traveling change-over valve from the first state to thesecond state in correspondence to modified example in thestraight-traveling degree.

The controller is configured or programmed to change a timing forshifting the traveling change-over valve from the first state to thesecond state in correspondence to modified example in thestraight-traveling degree.

The working machine further includes a first traveling device providedon a left portion of the machine body, and a second traveling deviceprovided on a right portion of the machine body. A first traveling motorconfigured to output a traveling power to the first traveling device anda second traveling motor configured to output a traveling power to thesecond traveling device are each provided as the traveling motor. Thetraveling pump is configured to rotate the first traveling motor and thesecond traveling motor. The traveling change-over valve is configured toshift the rotation speed of the first and second traveling motorsbetween the first speed and the second speed.

According to the working machine mentioned above, it is possible toeasily reduce a shift shock.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing a hydraulic system (a hydraulic circuit) for aworking machine according to a first embodiment.

FIG. 2 is a view showing a relationship between a rotation speed of aprime mover and switching of a traveling motor according to a case wherethe traveling motor is decelerated.

FIG. 3 is a view showing a relationship between the rotation speed ofthe prime mover and switching of the traveling motor according to thecase where the traveling motor is decelerated.

FIG. 4 is a view showing a modified example where an operation device isreplaced by another operation device such as a joy stick configured toelectrically operate.

FIG. 5 is a view showing a hydraulic system (a hydraulic circuit) for aworking machine according to a second embodiment.

FIG. 6 is a view showing a relationship between a straight-travelingdegree SV and a reduction amount ΔF11.

FIG. 7 is a view showing a relationship between a rotation speed of aprime mover and switching of a traveling motor according to a case wherethe traveling motor is decelerated.

FIG. 8 is a view showing a relationship between the rotation speed ofthe prime mover and switching of the traveling motor according to thecase where the traveling motor is decelerated.

FIG. 9 is a view showing a modified example where an operation device isreplaced by another operation device such as a joy stick configured toelectrically operate.

FIG. 10 is a side view showing a track loader that is an example of theworking machine.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment of a hydraulic system for a working machine andthe working machine having the hydraulic system will be described belowwith reference to drawings.

FIG. 10 is a side view of a working machine according to an embodiment.FIG. 10 shows a compact track loader as an example of the workingmachine. However, the working machine according to the embodiment is notlimited to the compact track loader. The working machine may be anothertyped loader, such as a skid steer loader. In addition, the workingmachine may be a working machine other than the loader working machine.

As shown in FIG. 10 , the working machine 1 includes a machine body 2, acabin 3, a working device 4, and a traveling device 5. In theembodiment, a direction corresponding to a forward direction from adriver siting on a driver seat 8 of the working machine 1 (leftward inFIG. 10 ) is referred to as “front” or “forward”, a rearward directionfrom the driver (rightward in FIG. 10 ) is referred to as “rear” or“rearward”, a leftward direction from the driver (a front surface sideof FIG. 10 ) is referred to as “left” or “rearward”, and a rightwarddirection from the driver (a back surface side of FIG. 10 ) is referredto as “right” or “rightward”. A horizontal direction orthogonal to afore-and-aft direction is referred to as a machine width direction. Adirection from the center of the machine body 2 to the right or left isreferred to as a machine outward direction. In other words, the machineoutward direction is one machine width direction away from the machinebody 2. A direction opposite to the machine outward direction isreferred to as a machine inward direction. In other words, the machineinward direction is the other machine width direction approaching themachine body 2.

The cabin 3 is mounted on the machine body 2. The cabin 3 incorporatesthe driver seat 8. The working device 4 is attached to the machine body2. The traveling device 5 is disposed outside the machine body 2. Aprime mover 32 is mounted on a rear inside portion of the machine body2.

The working device 4 has booms 10, a working tool (bucket 11, forexample), lift links 12, control links 13, boom cylinders 14, and bucketcylinders 15.

The booms 10 are arranged on right and left sides of the cabin 3swingably up and down. The working tool is, for example, a bucket 11.The bucket 11 is disposed on a tip portion (front end portion) of theboom 10 and is swingable up and down. The lift links 12 and the controllinks 13 support base portions (that is, rear portions) of the booms 10so that the booms 10 can be swung up and down. The boom cylinders 14 areextended and contracted to lift and lower the booms 10. The bucketcylinders 15 are extended and contracted to swing the bucket 11 up anddown.

Front portions of the right and left booms 10 are connected to eachother by a deformed connecting pipe. Base portions (that is, rearportions) of the booms 10 are connected to each other by a circularconnecting pipe.

The lift links 12, control links 13, and boom cylinders 14 are arrangedon right and left sides of the machine body 2 to correspond to the rightand left booms 10.

The lift links 12 are extended vertically from rear portions of the baseportions of the booms 10. Upper portions (that is, one ends) of the liftlinks 12 are pivotally supported on the rear portion of the baseportions of the booms 10 via respective pivot shafts 16 (referred to asfirst pivot shafts) rotatably around their lateral axes. In addition,lower portions (that is, the other ends) of the lift links 12 arepivotally supported on a rearward portion of the machine body 2 viarespective pivot shafts 17 (referred to as second pivot shafts)rotatably around their lateral axes. The second pivot shafts 17 areprovided below the first pivot shafts 16.

Upper portions of the boom cylinders 14 are pivotally supported viarespective pivot shafts 18 (referred to as third pivot shafts) rotatablyaround their lateral axes. The third pivot shafts 18 are provided at thebase portions of the booms 10, especially, at front portions of the baseportions. Lower portions of the boom cylinders 14 are pivotallysupported respective pivot shafts 19 (referred to as fourth pivotshafts) rotatably around their lateral axes. The fourth pivot shafts 19are provided closer to a lower portion of the rear portion of themachine body 2 and below the third pivot shafts 18.

The control links 13 are provided in front of the lift links 12. Oneends of the control links 13 are pivotally supported via respectivepivot shafts 20 (referred to as fifth pivot shafts) rotatably aroundtheir lateral axes. The fifth pivot shafts 20 are provided on themachine body 2 forward of the lift links 12. The other ends of thecontrol links 13 are pivotally supported via respective pivot shafts 21(referred to as sixth pivot shafts) rotatably around their lateral axes.The sixth pivot shafts 21 are provided on the booms 10 forwardly upwardfrom the second pivot shafts 17.

By extending and contracting the boom cylinders 14, the booms 10 areswung up and down around the first pivot shafts 16 with the baseportions of the booms 10 supported by the lift links 12 and the controllinks 13, thereby lifting and lowering the tip end portions of the booms10. The control links 13 are swung up and down around the fifth pivotshafts 20 by the vertical swinging of the booms 10. The lift links 12are swung back and forth around the second pivot shafts 17 by thevertical swinging of the control links 13.

An alternative working tool instead of the bucket 11 can be attached tothe front portions of the booms 10. For example, the alternative workingtool is an attachment (that is, an auxiliary attachment) such as ahydraulic crusher, a hydraulic breaker, an angle broom, an earth auger,a pallet fork, a sweeper, a mower, a snow blower, or the like.

A connecting member 50 is provided at the front portion of the left boom10. The connecting member 50 is a device configured to connect ahydraulic equipment attached to the auxiliary attachment to a firstpiping member such as a pipe provided on the boom 10. Specifically, thefirst piping member can be connected to one end of the connecting member50, and a second piping member connected to the hydraulic equipment ofthe auxiliary attachment can be connected to the other end. In thismanner, an operation fluid flowing in the first piping member passesthrough the second piping member and is supplied to the hydraulicequipment.

The bucket cylinders 15 are arranged respectively closer to the frontportions of the booms 10. The bucket cylinders 15 are extended andcontracted to swing the bucket 11.

Each of the traveling devices 5 (first traveling device 5L and secondtraveling device 5R) on the left or right portion is a crawler type(including semi-crawler type) traveling device in the presentembodiment. A wheel-type traveling device having front wheels and rearwheels may also be adopted.

The prime mover 32 is an internal combustion engine such as a dieselengine, a gasoline engine, an electric motor, or the like. In theembodiment, the prime mover 32 is the diesel engine, but is not limitedthereto.

Next, the hydraulic system for the working machine according to a firstembodiment will be described.

As shown in FIG. 1 , the hydraulic system for the working machine iscapable of driving the traveling devices 5. The hydraulic system for theworking machine includes a first traveling pump 53L, a second travelingpump 53R, a first traveling motor 36L and a second traveling motor 36R.

The first traveling pump 53L and the second traveling pump 53R are pumpsconfigured to be driven by a power of the prime mover 32. Specifically,the first traveling pump 53L and the second traveling pump 53R are swashplate type variable displacement axial pumps configured to be driven bythe power of the prime mover 32. Each of the first traveling pump 53Land the second traveling pump 53R includes pressure-receiving portions53 a and 53 b to which a pilot pressure is applied. An angle of theswash plate is changed by the pilot pressure applied to thepressure-receiving portions 53 a and 53 b. By changing the angle of theswash plate, outputs (delivering amounts of hydraulic fluid) anddelivery directions of the first traveling pump 53L and the secondtraveling pump 53R can be changed.

The first traveling pump 53L and the first traveling motor 36L areconnected to each other by a circulation fluid line 57 h so that thehydraulic fluid delivered by the first traveling pump 53L is supplied tothe first traveling motor 36L through the circulation fluid line 57 h.The second traveling pump 53R and the second traveling motor 36R areconnected to each other by a circulation fluid line 57 i so that thehydraulic fluid delivered by the second traveling pump 53R is suppliedto the second traveling motor 36R through the circulation fluid line 57i.

The first traveling motor 36L is a motor configured to transmit a powerto a drive shaft of the traveling device 5 disposed on the left portionof the machine body 2. The first traveling motor 36L is configured to berotated by hydraulic fluid delivered from the first traveling pump 53Land to change its rotation speed (number of rotations) according to aflow rate of the hydraulic fluid. A swash plate change-over cylinder 37Lis connected to the first traveling motor 36L, so that the rotationspeed (number of rotations) of the first traveling motor 36L can bechanged by extending and contracting the swash plate change-overcylinder 37L in one and the other directions. That is, when the swashplate change-over cylinder 37L is contracted, the rotation speed of thefirst traveling motor 36L is set to a low speed stage (first speed), andwhen the swash plate change-over cylinder 37L is extended, the rotationspeed of the first traveling motor 36L is set to a high speed stage(second speed). That is, the rotation speed of the first traveling motor36L can be changed between the first speed that is the low speed stageand the second speed that is the high speed stage.

The second traveling motor 36R is a motor configured to transmit a powerto a drive shaft of the traveling device 5 disposed on the right portionof the machine body 2. The second traveling motor 36L is configured tobe rotated by hydraulic fluid delivered from the second traveling pump53R and to change its rotation speed (number of rotations) according toa flow rate of the hydraulic fluid. A swash plate change-over cylinder37R is connected to the second traveling motor 36R so that the rotationspeed (number of rotations) of the second traveling motor 36L also canbe changed by extending and contracting the swash plate change-overcylinder 37R in one and the other directions. That is, when the swashplate change-over cylinder 37R is contracted, the rotation speed of thesecond traveling motor 36R is set to a low speed stage (first speed),and when the swash plate change-over cylinder 37R is extended, therotation speed of the second traveling motor 36R is set to a high speedstage (second speed). That is, the rotation speed of the secondtraveling motor 36L can be changed between the first speed that is thelow speed stage and the second speed that is the high speed stage.

As shown in FIG. 1 , the hydraulic system for the working machine has atraveling change-over valve 34. The traveling change-over valve 34 isconfigured to change the rotation speed (number of rotations) of thetravel motors (first traveling motor 36L and second traveling motor 36R)between a first state where the rotation speed becomes the first speedand a second state where the rotation speed becomes the second speed.The traveling change-over valve 34 includes first change-over valves 71Land 71R and a second switching valve 72.

The first change-over valve 71L is a two-position change-over valveconnected via a fluid line to the swashplate change-over cylinder 37L ofthe first traveling motor 36L and configured to be switched between thefirst position 71L1 and the second position 71L2. The first change-overvalve 71L, when set at the first position 71L1, contracts the swashplate change-over cylinder 37L, and when set at the second position71L2, extends the swash plate change-over cylinder 37L.

The first change-over valve 71R is a two-position change-over valveconnected via a fluid line to the swashplate change-over cylinder 37R ofthe second traveling motor 36R and configured to be switched between thefirst position 71R1 and the second position 71R2. The first change-overvalve 71R, when set at the first position 71R1, contracts the swashplate change-over cylinder 37R, and when set at the second position71R2, extends the swash plate change-over cylinder 37R.

The second switching valve 72 is a solenoid valve that switches thefirst change-over valve 71L and the first change-over valve 71R, and isconfigured as a two-position switching valve shiftable based onmagnetization between a first position 72 a and a second position 72 b.The second switching valve 72 is connected to the first change-overvalve 71L and the first change-over valve 71R via a fluid line 41. Thesecond switching valve 72, when set at the first position 72 a, switchesthe first change-over valves 71L and 71R to the first positions 71L1 and71R1, and when set at the second position 72 b, switches the firstchange-over valves 71L and 71R to the second positions 71L2 and 71R2.

That is, when the second change-over valve 72 is set at the firstposition 72 a, the first change-over valve 71L at the first position71L1, and the first change-over valve 71R at the first position 71R1,the traveling change-over valve 34 enters the first state, and therotation speed of the traveling motors (first traveling motor 36L andsecond traveling motor 36R) is set to the first speed. When the secondswitching valve 72 is set at the second position 72 b, the firstchange-over valve 71L at the second position 71L2, and the firstchange-over valve 71R at the second position 71R2, the travelingchange-over valve 34 enters the second state, and the rotation speed ofthe traveling motors (first traveling motor 36L and second travelingmotor 36R) is set to the second speed.

Accordingly, the traveling change-over valve 34 allows the travelingmotors (first traveling motor 36L and second traveling motor 36R) toswitch between the first speed that is the low speed stage and thesecond speed that is the high speed stage.

The switching between the first speed and the second speed in thetraveling motors can be performed by a change-over unit. The change-overunit is, for example, a change-over switch 61 connected to thecontroller 60 and can be operated by an operator or the like. Thechange-over unit (change-over switch 61) can be switched between anacceleration state where the rotation speed of the traveling motors isswitched from the first speed (first state) to the second speed (secondstate) and a deceleration state where the rotation speed of thetraveling motors is switched from the second speed (second state) to thefirst speed (first state).

The controller 60 is constituted of a CPU, an MPU or anothersemiconductor, electrical/electronic circuits, or the like. Thecontroller 60 switches the traveling change-over valve 34 based on aswitching operation of the change-over switch 61. The change-over switch61 is a push switch. When the change-over switch 61 is pressed, forexample, under a state where the traveling motor is at the first speed,an instruction to set the traveling motors to the second speed state (aninstruction to set the traveling change-over valve 34 to the secondstate) is output to the controller 60. In addition, when the change-overswitch 61 is pressed under a state where the travel motors are at thesecond speed, an instruction to set the travel motors to the first speed(an instruction to set the traveling change-over valve 34 to the firststate) is output to the controller 60. The change-over switch 61 may bea push switch that can be held as being turned ON and OFF, and when theswitch is turned OFF, an instruction to hold the traveling motors withtheir rotation speed set at the first speed is output to the controller60, and when the switch is turned ON, an instruction to hold thetraveling motors with their rotation speed set at the second speed isoutput to the controller 60.

When the controller 60 obtains the instruction to set the travelingchange-over valve 34 into the first state, the controller 60demagnetizes a solenoid of the second switching valve 72 to set thetraveling change-over valve 34 in the first state. When the controller60 obtains the instruction to set the traveling change-over valve 34into the second state, the controller 60 magnetizes the solenoid of thesecond switching valve 34 to set the traveling change-over valve 34 tobe in the second state by magnetizing a solenoid of the second switchingvalve 72.

The hydraulic system for the working machine is provided with a firsthydraulic pump P1, a second hydraulic pump P2 and an operation device(traveling-operation device) 54. The first hydraulic pump P1 is a pumpconfigured to be driven by power of the prime mover 32 and includes aconstant displacement gear pump. The first hydraulic pump P1 is capableof delivering operation fluid stored in a tank 22. Specifically, thefirst hydraulic pump P1 delivers operation fluid that is mainly used forcontrol. For convenience of explanation, the tank 22 that storesoperation fluid may be referred to as an operation fluid tank. Of theoperation fluid delivered from the first hydraulic pump P1, theoperation fluid used for control is referred to as pilot fluid, and apressure of the pilot fluid is referred to as a pilot pressure.

The second hydraulic pump P2 is a pump configured to be driven by powerof the prime mover 32, and includes a constant displacement gear pump.The second hydraulic pump P2 is capable of delivering operation fluidstored in the tank 22 and, for example, supplies the operation fluid tofluid lines of a working system. For example, the second hydraulic pumpP2 supplies operation fluid to control valves (that is, flow-ratecontrol valves) that control the boom cylinders 14 for operating thebooms 10, the bucket cylinders 15 for operating the bucket, and anauxiliary hydraulic actuator for operating the auxiliary hydraulicactuator.

The traveling operation device 54 (operation device) operates thetraveling pumps (first traveling pump 53L and second traveling pump 53R)and can change angles of the swash plates of the traveling pumps (swashplate angles). The traveling operation device 54 includes an operationmember 59, such as an operation lever, and a plurality of operationvalves 55.

The operation member 59 is an operating member supported on theoperation valves 55 and configured to swing in a lateral direction(machine width direction) or a fore-and-aft direction. That is, withreference to the neutral position N, the operation member 59 is operableto the right and to the left from a neutral position N, and to the frontand to the rear from the neutral position N. In other words, theoperation member 59 is capable of being swung in at least fourdirections with respect to the neutral position N. For convenience ofexplanation, the forward and backward directions, that is, thefore-and-aft directions, may be referred to as first directions. Inaddition, the rightward and leftward directions, that is, the lateraldirections (that is, the machine width directions), are may be referredto as second directions.

The plurality of operation valves 55 are operated by a common, i.e., asingle, operation member 59. The plurality of operation valves 55 areactuated based on the swinging of the operation member 59. A deliveryfluid line 40 is connected to the plurality of operation valves 55, sothat operation fluid (that is, pilot fluid) from the first hydraulicpump P1 can be supplied to the operation valves 55 through the outputfluid line 40. The plurality of operation valves 55 include an operationvalve 55A, an operation valve 55B, an operation valve 55C, and anoperation valve 55D.

When the operation member 59 is swung forward (one of the oppositefore-and-aft directions (first directions)) (this operation is referredto as “forward operation”), the operation valve 55A changes a pressureof hydraulic fluid output according to the operation amount of theoperation member 59 in the forward operation. When the operation member59 is swung backward (the other of the opposite fore-and-aft directions(first directions)) (this operation is referred to as “backwardoperation”), the operation valve 55B changes a pressure of hydraulicfluid output according to the operation amount of the operation member59 in the backward operation. When the operation member 59 is swungrightward (one of the opposite lateral directions (second directions))(this operation is referred to as “rightward operation”), the operationvalve 55C changes a pressure of hydraulic fluid output according to theoperation amount of the operation member 59 in the rightward operation.When the operation member 59 is swung leftward (the other of theopposite lateral directions (second directions)) (this operation isreferred to as “leftward operation”), the operation valve 55D changes apressure of hydraulic fluid output according to the operation amount ofthe operation member 59 in the leftward operation.

The plurality of operation valves 55 are fluidly connected to thetraveling pumps (first traveling pump 53L, second traveling pump 53R)via a traveling fluid line 45. In other words, the traveling pumps(first traveling pump 53L and second traveling pump 53R) are hydraulicdevices that can be actuated by hydraulic fluid output from theoperation valves 55 (operation valve 55A, operation valve 55B, operationvalve 55C, and operation valve 55D).

The traveling fluid line 45 includes a first traveling fluid line 45 a,a second traveling fluid line 45 b, a third traveling fluid line 45 c, afourth traveling fluid line 45 d, and a fifth traveling fluid line 45 e.The first traveling fluid line 45 a is a fluid line connected to apressure-receiving portion 53 a of the travelling pump 53L. The secondtraveling fluid line 45 b is a fluid line connected to apressure-receiving portion 53 b of the traveling pump 53L. The thirdtraveling fluid line 45 c is a fluid line connected to apressure-receiving portion 53 a of the travelling pump 53R. The fourthtraveling fluid line 45 d is a fluid line connected to apressure-receiving portion 53 b of the traveling pump 53R. The fifthtraveling fluid line 45 e is a fluid line that connects the operationvalves 55 to the first traveling fluid line 45 a, the second travelingfluid line 45 b, the third traveling fluid line 45 c, and the fourthtraveling fluid line 45 d.

When the operation member 59 is swung forward (in a direction of anarrowed line A1 in FIG. 1 ), the operation valve 55A is operated and apilot pressure is output from the operation valve 55A. This pilotpressure is applied to the pressure-receiving portion 53 a of the firsttraveling pump 53L via the first traveling fluid line 45 a, and to thepressure-receiving portion 53 a of the second traveling pump 53R via thethird traveling fluid line 45 c. In this manner, the swash plate anglesof the first and second traveling pumps 53L and 53R are set so that thefirst and second traveling motors 36L and 36R rotate normally (forwardtraveling rotation), and the working machine 1 travels straight forward.

When the operation member 59 is swung backward (in a direction of anarrowed line A2 in FIG. 1 ), the operation valve 55B is operated and apilot pressure is output from the operation valve 55B. This pilotpressure is applied to the pressure-receiving portion 53 b of the firsttraveling pump 53L via the second traveling fluid line 45 b, and to thepressure-receiving portion 53 b of the second traveling pump 53R via thefourth traveling fluid line 45 d. In this manner, the swash plate anglesof the first and second traveling pumps 53L and 53R are so that thefirst and second traveling motors 36L and 36R rotate reversely (backwardtraveling rotation), and the working machine 1 travels straightbackward.

When the operation member 59 is swung rightward (in a direction of anarrowed line A3 in FIG. 1 ), the operation valve 55C is operated and apilot pressure is output from the operation valve 55C. This pilotpressure is applied to the pressure-receiving portion 53 a of the firsttraveling pump 53L via the first traveling fluid line 45 a, and to thepressure-receiving portion 53 b of the second traveling pump 53R via thefourth traveling fluid line 45 d. In this manner, the swash plate anglesof the first and second traveling pumps 53L and 53R are set so that thefirst traveling motor 36L rotates normally and the second travelingmotor 36R rotates reversely, and the working machine 1 turns to theright.

When the operation member 59 is swung leftward (in a direction of anarrowed line A4 in FIG. 1 ), the operation valve 55D is operated and apilot pressure is output from the operation valve 55D. This pilotpressure is applied to the pressure-receiving portion 53 a of the secondtraveling pump 53R via the third traveling fluid line 45 c, and to thepressure-receiving portion 53 b of the first traveling pump 53L via thesecond traveling fluid line 45 b. In this manner, the swash plate anglesof the first and second traveling pumps 53L and 53R are set so that thefirst traveling motor 36L rotates reversely and the second travelingmotor 36R rotates normally, and the working machine 1 turns to the left.

When the operation member 59 is swung in an oblique direction, therotations of the first traveling motor 36L and the second travelingmotor 36R are controlled in direction and speed by the differentialpressure between the pilot pressures acting on the pressure-receivingportion 53 a and the pressure-receiving portion 53 b, and the workingmachine 1 turns right or left while traveling forward or backward.

That is, when the operation member 59 is swung diagonally forwardlyleftward, the working machine 1 turns left while traveling forward at aspeed corresponding to the swing angle of the operation member 59. Whenthe operation member 59 is swung diagonally forwardly rightward, theworking machine 1 turns right while traveling forward at a speedcorresponding to the swing angle of the operation member 59. When theoperation member 59 is swung diagonally backward leftward, the workingmachine 1 turns left while traveling backward at a speed correspondingto the swing angle of the operation member 59. When the operation member59 is swung diagonally backwardly rightward, the working machine 1 turnsright while traveling backward at a speed corresponding to the swingangle of the operation member 59.

The controller 60 is connected to an accelerator 65 operable to set atarget rotation speed of the prime mover 32. The accelerator 65 isprovided in the vicinity of the driver's seat 8. The accelerator 65 is aswingably-supported accelerator lever, a swingably-supported acceleratorpedal, a rotatably-supported accelerator knob, a slidably-supportedaccelerator slider, or the like. The accelerator 65 is not limited tothe examples mentioned above. The controller 60 is connected to arotation detector 66 configured to detect an actual rotation speed ofthe prime mover 32. By means of the rotation detector 66, the controller60 is capable of knowing an actual rotation speed of the prime mover 32.Based on an operation amount of the accelerator 65, the controller 60sets a target rotation speed and controls an actual rotation speed so asto reach the set target rotation speed.

The controller 60 performs a shock-mitigation control to reduce a primemover speed when switching the traveling change-over valve 34 from thesecond state (second speed) to the first state (first speed), i.e., whenreducing the rotation speed of the traveling motors from the secondspeed to the first speed.

The shock-mitigation control in deceleration will be described in detailbelow.

FIG. 2 shows a relationship between a rotation speed of the prime mover(target rotation speed W10, actual rotation speeds W12 a, W12 b, W12 c)and the speed-switching of the traveling motors in the shock-mitigationcontrol during deceleration. As shown in FIG. 2 , the controller 60 setsa reduction amount ΔF1 of a rotation speed of the prime mover in theshock-mitigation control based on a drop amount ΔD1 that is a differencebetween the target rotation speed W10 of the prime mover 32 and theactual rotation speeds W12 a, W12 b and W12 c of the prime mover 32, andreduces a shift shock according to the reduction amount ΔF1.

Specifically, it is supposed that the change-over switch (change-overSW) 61 is operated at time point Q11, and the controller 60 obtains adeceleration instruction (first speed instruction) to change the speedstage from the second state (second speed) to the first state (firstspeed).

When the controller 60 obtains the first speed instruction, thecontroller 60 obtains drop amount ΔD1 (ΔD1 a, ΔD1 b, ΔD1 c) bysubtracting each of the actual rotation speeds W12 a, W12 b, and W12 cfrom the target rotation speed W10. When the controller 60 obtains thedrop amount ΔD1 (ΔD1 a, ΔD1 b, ΔD1 c), the controller 60 sets thereduction amount ΔF1 (ΔF1 a, ΔF1 b, ΔF1 c) based on the drop amount ΔD1(ΔD1 a, ΔD1 b, ΔD1 c). In setting the reduction amount ΔF1, thecontroller 60 increases the reduction amount ΔF1 when the drop amountΔD1 is small, and decreases the reduction amount ΔF1 when the dropamount ΔD1 is large.

For example, at a time point Q11, a drop amount ΔD1 a is obtained, andthe controller 60 sets a reduction amount ΔFla. Alternatively, at timepoint Q11, a drop amount ΔD1 b is obtained, the controller 60 sets areduction amount ΔF1 b. Alternatively, at time point Q11, a drop amountΔD1 c is obtained, the controller 60 sets a reduction amount ΔF1 c.

Thus, the controller 60 determines the reduction amount ΔF1 (ΔF1 a, ΔF1b, ΔF1 c) depending on the drop amount ΔD1 (ΔD1 a, ΔD1 b, ΔD1 c)obtained at the time point Q11. For example, a characteristic mapshowing a relationship between the drop amount ΔD1 and the reductionamount ΔF1 is previously stored in the storage device. The drop amountΔD1 and the reduction amount ΔF1 are inversely proportional to eachother. The controller 60 determines the reduction amount ΔF1corresponding to the drop amount ΔD1 by using the characteristic map.The controller 60 may make the reduction amount ΔF1 greater than aspecified amount when the drop amount ΔD1 is less than or equal to thespecified amount, or may make the reduction amount ΔF1 less than thespecified amount when the drop amount ΔD1 is greater than or equal tothe specified amount.

Once the reduction amount ΔF1 is set, the controller 60 sets the valueobtained by subtracting the reduction amount ΔF1 (ΔFla, ΔF1 b, ΔF1 c)from the actual rotation speed W12 a, W12 b or W12 c to the mitigationvalue W14 a, W14 b or W14 c of the prime mover in the shock-mitigationcontrol. For example, when the drop amount is ΔD1 a, the controller 60sets, as the mitigation value, the value W14 a which is obtained bysubtracting the reduction amount ΔFla from the actual rotation speed W12a. When the drop amount is ΔD1 b, the controller 60 sets, as themitigation value, the value W14 b which is obtained by subtracting thereduction amount ΔF1 b from the actual rotation speed W12 b. When thedrop amount is ΔD1 c, the controller 60 sets, as the mitigation value,the value W14 c which is obtained by subtracting the reduction amountΔF1 c from the actual rotation speed W12 c.

After setting the mitigation values W14 a, W14 b and W14 c, thecontroller 60 decreases an actual rotation speed of the prime moveruntil the reduced actual rotation speed of the prime mover reaches themitigation value W14 a, W14 b or W14 c.

Specifically, when the drop amount is ΔD1 a at a time point Q11, thecontroller 60 decreases an actual rotation speed of the prime movertoward the mitigation value W14 a, as descending along a line W11 a.When the reduced actual rotation speed of the prime mover reaches themitigation value W14 a at a time point Q12 a, the controller 60 outputsa signal to demagnetize the solenoid of the traveling change-over valve34 to switch the traveling change-over valve (change-over valve) 34 fromthe second state (second speed) to the first state (first speed), asshown in a graph of the change-over valve control signal correspondingto the line W11 a. After the time point Q12 a, an actual rotation speedis restored to a target rotation speed (original actual rotation speedW12 a), as rising along the line W11 a.

Alternatively, when the drop amount is ΔD1 b at a time point Q11, thecontroller 60 decreases an actual rotation speed of the prime movertoward the mitigation value W14 b, as descending along a line W11 b.When the reduced actual rotation speed of the prime mover reaches themitigation value W14 b at a time point Q12 b, the controller 60 outputsa signal to demagnetize the solenoid of the traveling change-over valve34 to switch the traveling change-over valve (change-over valve) 34 fromthe second state (second speed) to the first state (first speed), asshown in a graph of the change-over valve control signal correspondingto the line W11 b. After the time point Q12 b, an actual rotation speedis restored toward a target rotation speed (original actual rotationspeed W12 b), rising along the line W11 b.

Alternatively, when the drop amount is ΔD1 c at a time point Q11, thecontroller 60 decreases an actual rotation speed of the prime movertoward the mitigation value W14 c, as descending along a line W11 c.When the reduced actual rotation speed of the prime mover reaches themitigation value W14 c at a time point Q12 c, the controller 60 outputsa signal to demagnetize the solenoid of the traveling change-over valve34 to switch the traveling change-over valve (change-over valve) 34 fromthe second state (second speed) to the first state (first speed), asshown in a graph of the change-over valve control signal correspondingto the line W11 c. After the time point Q12 c, an actual rotation speedis restored to a target rotation speed (original actual rotation speedW12 c), as rising along the line W11 c.

Now, focusing on each of mitigation periods Ta, Tb and Tc from the timepoint Q11, which is the starting point of the reduction of the actualrotation speed of the prime mover, to each of the time points Q12 a, Q12b and Q12 c, which is the end point of the reduction of the actualrotation speed of the prime mover, that is, the mitigation period Ta, Tbor Tc for which the actual rotation speed of the prime mover is reduceduntil reaching the corresponding mitigation value W14 a, W14 b or W14 c,the controller 60 keeps a constant reduction rate of the actual rotationspeed of the prime mover defined as a first reduction rate. That is, foreach of the mitigation periods Ta, Tb and Tc, the controller 60 keeps aconstant slope of each of the respective lines W11 a, W11 b and W11 c.

The traveling change-over valve 34 is switched between the first stateand the second state at any of the different time points Q12 a, Q12 band Q12 c. Therefore, the controller 60 is configured to change a timingfor switching the traveling change-over valve 34 between the first stateand the second state in correspondence to the drop amount D1.

In the above-described embodiment, for each of the mitigation periodsTa, Tb and Tc, the actual rotation speed of the prime mover is reducedat a reduction rate that is kept constant for the correspondingmitigation period Ta, Tb or Tc from the start to the end. Alternatively,the reduction rate may be changed at an intermediate point of themitigation period.

FIG. 3 shows a modified example in which the reduction rate of an actualrotation speed of the prime mover is changed at an intermediate point ofthe mitigation period Ta.

When the controller 60 obtains the first speed instruction andcalculates the mitigation value W14 a based on the drop amount ΔD1 a,the controller 60 sets a reduction rate of the prime mover, in a section(first section) Ta1 defined as a first part of the mitigation period Tafrom the start to the intermediate point, to a second reduction rate,and sets the reduction rate of the prime mover, in a section (secondsection) Ta2 defined as a second part of the mitigation period Ta fromthe intermediate point to the end, to a third reduction rate, as shownin FIG. 3 . That is, while the line W11 a indicates variation of anactual rotation speed of the prime mover for the mitigation period Ta,the controller 60 sets the second reduction rate in the first sectionTa1 based on a slope of a line W11 a 1 in the line W11 a, and the thirdreduction rate for the second section Ta2 in the mitigation period Tabased on a slope of a line W11 a 2 in the line W11 a. The controller 60sets the second reduction rate (slope of line W11 a 1) as a valuegreater than the third reduction rate (slope of line W11 a 2).

In the modified example, the second and third reduction rates settingbased on the line W11 a is described. However, other lines W11 b and W11c may be used to set the second and third reduction rates in the sameway as the setting with the line W11 a. In this case, the drop amountΔD′ a should be read as the drop amount ΔD1 b or ΔD1 c, the mitigationvalue W14 a should be read as the mitigation value W14 b or W14 c, themitigation period Ta should be read as the mitigation period Tb or Tc,the line W11 a should be read as the line W11 b or W11 c, the line W11 a1 should be read as a line W11 b 1 or W11 c 1, the line W11 a 2 shouldbe read as a line W11 b 2 or W11 c 2, the first section Ta1 should beread as a first section Tb1 or Tc1, and the second section Ta2 should beread as a second section Tb2 or Tc2.

In the above-mentioned embodiment, the traveling operation device 54 ishydraulically operated to change pilot pressures acting on the travelingpumps (first traveling pump 53L and second traveling pump 53R) by meansof the operation valves 55. However, as shown in FIG. 4 , the travelingoperation device 54 may be an electrically operable device.

As shown in FIG. 4 , the traveling operation device 54 is provided withthe operation member 59 swingable in the lateral direction (machinewidth direction) or in the fore-and-aft direction, and proportionalsolenoid valves serving as the operation valves 55 (including theoperation valves 55A, 55B, 55C, and 55D). The controller 60 is connectedto an operation detection sensor which detects an operation amount andoperational direction of the operation member 59. The controller 60controls the operation valves 55 (including the operation valves 55A,55B, 55C, and 55D) based on the operation amount and operationaldirection detected by the operation detection sensor.

When the operation member 59 is operated to the front (the direction A1,see FIG. 1 ), the controller 60 outputs a control signal to theoperation valves 55A and 55C to swing the swash plates of the firsttraveling pump 53L and the second traveling pump 53R in the directionfor normally rotating the first traveling motor 36L and the secondtraveling motor 36R (for forward traveling).

When the operation member 59 is operated to the rear (the direction A2,see FIG. 1 ), the controller 60 outputs a control signal to theoperation valves 55B and 55D to swing the swash plates of the firsttraveling pump 53L and the second traveling pump 53R in the directionfor reversely rotating the motors (for backward traveling).

When the operation member 59 is operated to the right (the direction A3,see FIG. 1 ), the controller 60 outputs a control signal to theoperation valve 55A and the operation valve 55D to swing the swash plateof the first traveling pump 53L in the direction for normally rotatingthe first traveling motor 36L and to swing the swash plate of the secondtraveling pump 53R in the direction for reversely rotating the secondtraveling motor 36R.

When the operation member 59 is operated to the left (the direction A4,see FIG. 1 ), the controller 60 outputs a control signal to theoperation valve 55B and the operation valve 55C to swing the swash plateof the first traveling pump 53L in the direction for reversely rotatingthe first traveling motor 36L and to swing the swash plate of the secondtraveling pump 53R in the direction for normally rotating the secondtraveling motor 36R.

The working machine 1 includes the prime mover 32, the traveling pumps53L and 53R driven by power of the prime mover 32 so as to deliveroperation fluid, the traveling motors 36L and 36R configured to berotated by the operation fluid delivered from the traveling pumps 53Land 53R, the traveling motors 36L and 36R having a rotation speedshiftable between the first speed and the second speed that is higherthan the first speed, the machine body provided thereon with the primemover 32, the traveling pumps 53L and 53R and the traveling motors 36Land 36R, the traveling change-over valve 34 shiftable between the firststate where the rotation speed of the traveling motors 36L and 36R isset to the first speed and the second state where the rotation speed ofthe traveling motors 36L and 36R is set to the second speed, and thecontroller 60 configured or programmed to perform the shock-mitigationcontrol for reducing a rotation speed of the prime mover 32 when thetraveling change-over valve 34 is shifted from the second state to thefirst state. The controller 60 is configured or programmed to determinea reduction amount of rotation speed of the prime mover 32 reduced bythe shock mitigation control based on a drop amount ΔD1 defined as adifference between a target rotation speed of the prime mover 32 and anactual rotation speed of the prime mover 32.

According to this configuration, the reduction amount ΔF1 of a rotationspeed of the prime mover 32 is set according to a load on the primemover 32, i.e., the drop amount ΔD1 that is a difference between thetarget rotation speed of the prime mover 32 and an actual rotation speedof the prime mover 32. Accordingly, the actual rotation speed of theprime mover 32 can be reduced (lowered) in correspondence to the dropamount ΔD1 when the shock-mitigation control is performed, therebyimproving workability.

The controller 60 is configured or programmed to calculate a mitigationvalue of the prime mover 32 in the shock mitigation control bysubtracting the reduction amount ΔF1 from the actual rotation speed ofthe prime mover 32. According to this configuration, the actual rotationspeed of the prime mover 32 during deceleration to be set appropriately,even when the actual rotation speed of the prime mover 32 is dropped dueto the load.

When a period for the shock mitigation control until the actual rotationspeed of the prime mover 32 reduced by the shock mitigation controlreaches the mitigation value is defined as the mitigation period Ta, Tbor Tc, the controller 60 is configured or programmed to reduce theactual rotation speed of the prime mover at the first reduction ratekept constant for the mitigation period Ta, Tb or Tc from the startpoint thereof to the end point thereof. According to this configuration,the outputs of the traveling pumps 53L and 53R can be reduced smoothlyand gradually, so that the shift shock can be reduced more efficiently.

When a period for the shock mitigation control until the actual rotationspeed of the prime mover 32 reduced by the shock mitigation controlreaches the mitigation value is defined as the mitigation period Ta, Tbor Tc, the controller 60 is configured or programmed to reduce theactual rotation speed of the prime mover 32 at the second reduction ratefor the first part of the mitigation period Ta, Tb or Tc from the startpoint thereof to the intermediate point thereof, and to reduce theactual rotation speed of the prime mover 32 at the third reduction rateless than the second reduction rate for the second part of themitigation period Ta, Tb or Tc from the intermediate point thereof tothe end point thereof. According to this configuration, theresponsiveness of the traveling pumps 53L and 53R can be improved in theshock-mitigation control.

The controller 60 is configured or programmed to change a timing forshifting the traveling change-over valve 34 from the second state to thefirst state in correspondence to the drop amount ΔD1. According to thisconfiguration, a timing of the deceleration can be changed according toa load on the prime mover 32, thereby further improving the workability.

The working machine 1 further includes the change-over switch 61operable to issue a speed-shift instruction selected between anacceleration instruction to shift the rotation speed of the travelingmotors 36L and 36R from the first speed to the second speed and adeceleration instruction to shift the rotation speed of the travelingmotors 36L and 36R from the second speed to the first speed, and theaccelerator 65 operable to set a target rotation speed of the primemover 32. When the change-over switch 61 is operated to issue thespeed-shift instruction, the controller 60 is configured or programed toreduce the actual rotation speed of the prime mover 32 to the mitigationvalue determined based on the reduction amount ΔF1, and to shift thetraveling change-over valve 34 to either the first state or the secondstate in correspondence to the speed-shift instruction. According tothis configuration, the actual rotation speed of the prime mover 32 canbe sufficiently lowered before shifting speeds, thereby furtherimproving reduction of the shift shock.

The controller 60 is configured or programmed to determine the reductionamount ΔF1 based on the drop amount ΔD1 in such a way that the reductionamount becomes larger as the drop amount ΔD1 becomes smaller, and thereduction amount ΔF1 becomes smaller as the drop amount ΔD1 becomeslarger. According to this configuration, when the load applied to theprime mover 32 is small and there is a margin in output of the primemover 32, the reduction of the shift shock can be further improved.Moreover, when the load applied to the prime mover 32 is large and thereis a margin in the output of the prime mover 32, the reduction of theshift shock can be moderated so that the actual rotation speed of theprime mover 32 after the reduction of the shift shock (after shiftingspeed) can be restored earlier while reducing the shift shock.

The working machine 1 further includes the first traveling device 5Lprovided on a left portion of the machine body 2, and the secondtraveling device 5R provided on a right portion of the machine body 2.The first traveling motor configured to output a traveling power to thefirst traveling device 5L and the second traveling motor configured tooutput a traveling power to the second traveling device 5R are eachprovided as the traveling motors 36L and 36R, the traveling pumps 53Land 53R are configured to rotate the first traveling motor and thesecond traveling motor, and the traveling change-over valve 34 isconfigured to shift the rotation speed of the first and second travelingmotors between the first speed and the second speed. According to thisconfiguration, the shift shock can be reduced more smoothly in theworking machine 1 including the first traveling device 5L disposed onthe left portion of the machine body 2 and the second traveling device5R disposed on the right portion of the machine body 2.

The traveling change-over valve 34 is enough only if it is a valveconfigured to switch the traveling motors (first traveling motor 36L andsecond traveling motor 36R) between the first state for setting thefirst speed and the second state for setting the second speed. Thetraveling change-over valve 34 may be a proportional valve differentfrom a directional change-over valve.

The traveling motor may be a motor having a neutral position (neutral)between the first and second speeds.

The travel motors (first traveling motor 36L and second traveling motor36R) may be axial piston motors or radial piston motors. If thetravelling motor is a radial piston motor, it is configured so that thetraveling motors are set in the first speed when the motor capacitybecomes large, and the traveling motors are set in the second speed, andso that the traveling motors are set in the second speed when the motorcapacity becomes small.

Next, a hydraulic system for a working machine according to a secondembodiment will be described. For convenience of explanation, componentshaving the same functions as those of the first embodiment are given thesame reference numerals, and their description is omitted.

The controller 60 performs the shock-mitigation control when switchingthe traveling change-over valve 34 from the first state (first speed) tothe second state (second speed), that is, when increasing the rotationspeed of the travel motors by shifting the speed stage from the firstspeed to the second speed.

As shown in FIG. 5 , in the shock-mitigation control, the controller 60reduces the shock of the gear shift by controlling an opening degree ofan actuation valve 69. The actuation valve 69 is connected to a section40 a of a delivery fluid line 40 after branching and leading to thetraveling operation device 54, i.e., upstream of the operation valve 55.The actuation valve 69 may be connected to a portion of the travelingfluid line 45 downstream of the operation valve 55.

The actuation valve 69 is a proportional solenoid valve (proportionalvalve), having an opening degree which can be changed by a controlsignal output from the controller 60. The control signal is, forexample, a voltage, a current or the like. The actuation valve 69 is avalve whose opening degree increases as the control signal (voltage,current) output from the controller 60 becomes high, and decreases asthe control signal (voltage, current) becomes low.

That is, in shock-mitigation control, the controller 60 decreases theopening degree of the actuation valve 69 by changing the control signalto be output to the actuation valve 69.

When performing the shock-mitigation control, the controller 60 sets thereduction amount in the opening degree of the actuation valve 69 basedon a straight-traveling degree (straight traveling degree) of theworking machine 1 (machine body 2). The straight-traveling degree can bedetermined based on the pressure of the hydraulic fluid in the travelingfluid line 45.

As shown in FIG. 5 , at least one pressure detector 48 is connected tothe traveling fluid line 45 to detect a pressure (pilot pressure) ofhydraulic fluid in the traveling fluid line 45. The at least onepressure detector 48 includes a first pressure detector 48 a, a secondpressure detector 48 b, a third pressure detector 48 c, and a fourthpressure detector 48 d. The first pressure detector 48 a, the secondpressure detector 48 b, the third pressure detector 48 c, and the fourthpressure detector 48 d are connected to the controller 60.

The first pressure detector 48 a is a sensor capable of detecting afirst pilot pressure lf(t) which is a pressure of the hydraulic fluid inthe first traveling fluid line 45 a. The second pressure detector 48 bis a sensor capable of detecting a second pilot pressure lb(t) which isa pressure of the hydraulic fluid in the second traveling fluid line 45b. The third pressure detector 48 c is a sensor capable of detecting athird pilot pressure rf(t) which is a pressure of the hydraulic fluid inthe third traveling fluid line 45 c. The fourth pressure detector 48 dis a sensor capable of detecting a fourth pilot pressure rb(t) which isa pressure of the hydraulic fluid in the fourth traveling fluid line 45d.

As shown in Equations (1) and (2), the controller 60 obtains astraight-traveling degree S_(Bratio)(t) and a straight-traveling degreeS_(Fratio)(t) based on the first pilot pressure lf(t), the second pilotpressure lb(t), the third pilot pressure rf(t), and the fourth pilotpressure rb(t). When a ratio (rf(t)/lf(t)) is not within a predeterminedrange, the controller 60 sets larger one of the first pilot pressurelf(t) and the third pilot rf(t) as the first straight-traveling valuePv_(Bpivot). When the ratio (rb(t)/lb(t)) is not within thepredetermined range, the controller 60 sets larger one of the secondpilot lb(t) and the fourth pilot rb(t) as the second straight-travelingvalue PV_(Fpivot).

(Equation)

$\begin{matrix}{S_{F{{ratio}(t)}} = {\left( \frac{{rf}_{(t)} + {lf_{(t)}}}{2} \right)/{pv}_{Fpivot}}} & (1)\end{matrix}$ $\begin{matrix}{{S_{{Bratio}(t)} = {\left( \frac{{rb_{(t)}} + {lb_{(t)}}}{2} \right)/{pv}_{Bpivot}}},} & (2)\end{matrix}$wherepv _(Bpivot)=max(rf _((t)) ,lf _((t))),pv _(Fpivot)=max(rb _((t)),lb_((t)))

The controller 60 judges, based on the straight-traveling degreeS_(Bratio)(t) and the straight-traveling degree S_(Fratio)(t), whetherthe working machine 1 travels straight or not. For example, when thestraight-traveling degree S_(Bratio)(t) or the straight-traveling degreeS_(Fratio)(t) is so large as to exceed 1.0, the controller 60 considersthe working machine 1 (machine body 2) as traveling straight. When thestraight-traveling degree S_(Bratio)(t) or the straight-traveling degreeS_(Fratio)(t) is less than 1.0 and so small as close to zero, thecontroller 60 considers the working machine 1 as traveling in pivotalturn.

For convenience of explanation, the straight-traveling degreeS_(Bratio)(t) and the straight-traveling degree S_(Fratio)(t) are eachhereinafter simply referred to as “straight-traveling degree SV”.

The shock-mitigation control in acceleration is described in detailbelow.

As shown in FIG. 6 , when performing the shock-mitigation control, thecontroller 60 sets the reduction amount ΔF11 of the control signal inthe shock-mitigation control based on the straight-traveling degree SV.For example, the controller 60 increases the reduction amount ΔF11 asthe straight-traveling degree SV increases, and decreases the reductionamount ΔF11 as the straight-traveling degree SV decreases. In otherwords, the controller 60 increases the reduction amount ΔF11 when thestraight-traveling degree SV is large such that the working vehicle 1travels substantially straight, and the controller 60 decreases thereduction amount ΔF11 when the straight-traveling degree SV is smallsuch that the working machine 1 travels in substantially pivotal turn.

FIG. 7 is a view showing a relationship between the control values ofthe control signals, i.e., mitigation values W24 a, W24 b and W24 c, andthe speed-switching of the traveling motors under the shock-mitigationcontrol in acceleration.

It is supposed that, at a time point Q11, the change-over switch(change-over SW) 61 is operated and the controller 60 obtains anacceleration instruction (second speed instruction) to switch the speedstage from the first state (first speed) to the second state (secondspeed). When the controller 60 obtains the second speed instruction, thecontroller 60 calculates the straight-traveling degree SV and sets thereduction amount ΔF11 based on the calculated straight-traveling degreeSV.

As shown in FIG. 7 , for example, the controller 60 sets the reductionamount ΔF11 a when, at the time point Q11, the straight-traveling degreeSV is so large as to represent the traveling mode of the working machine1 as approximately straight traveling. Alternatively, the controller 60sets the reduction amount ΔF11 b when, at the time point Q11, thestraight-traveling degree SV is smaller than that corresponding to thestraight traveling and the traveling status is slightly closer to apivotal turn. Alternatively, the controller 60 sets the reduction amountΔF11 c when, at time point Q11, the straight-traveling degree SV is verysmall, and the traveling status is close to a pivotal turn.

Thus, the controller 60 sets the reduction amount ΔF11 (ΔF11 a, ΔF11 b,or ΔF11 c) according to the straight-traveling degree SV at the timepoint Q11.

After setting the reduction amount ΔF11, the controller 60 sets, as themitigation value W24 a, W24 b or W24 c in the shock-mitigation control,the values obtained by subtracting the reduction amount ΔF11 (ΔF11 a,ΔF11 b or ΔF11 c) from the control value (current control value) W22 aof the control signal immediately before the reduction. For example, thecontroller 60 sets a value W24 a, which is the value obtained bysubtracting a reduction amount ΔF11 a from the current control valueW22, as the mitigation value. Alternatively, the controller 60 sets, asthe mitigation value, a value W24 b, which is the value obtained bysubtracting a reduction amount ΔF11 b from the current control value W22a. Alternatively, the controller 60 sets, as the mitigation value, avalue W24 c, which is the value obtained by subtracting a reductionamount ΔF11 b from the current control value W22 a.

After setting the mitigation value W24 a, W24 b or W24 c, the controller60 reduces the control value to be output to the actuation valve 69until the control value reaches the mitigation value W24 a, W24 b or W24c.

Specifically, at the time point Q11, when the machine body 2 istravelling substantially straight, the controller 60 decreases thecontrol value to the mitigation value W24 a, as descending along a lineW31 a. When the control value reaches the mitigation value W24 a at atime point Q12 a as shown in a graph of the change-over valve controlsignal corresponding to the line W31 a, the controller 60 outputs asignal to magnetize the solenoid of the traveling change-over valve 34to switch the traveling change-over valve (switching valve) 34 from thefirst state (first speed) to the second state (second speed). After thetime point Q12 a, the control value is restored to the control value W22before the reduction, as rising along the line W31 a.

Alternatively, at the time point Q11, when the machine body 2 istravelling in a slightly approximate pivotal turn rather than travelingstraight, the controller 60 reduces the control value to the mitigationvalue W24 b, as descending along a line W31 b. When the control valuereaches the mitigation value W24 b at a time point Q12 b, as shown in agraph of the change-over valve control signal corresponding to the lineW31 b, the controller 60 outputs a signal to magnetize the solenoid ofthe traveling change-over valve 34 to switch the traveling change-overvalve (switching valve) 34 from the first state (first speed) to thesecond state (second speed). In addition, after the time point Q12 b,the control value is restored to the control value W22 before thereduction, as rising along the line W31 b.

Alternatively, at the time point Q11, when the machine body 2 istravelling in further approximate pivotal turn, the controller 60reduces the control value to the mitigation value W24 c, as descendingalong the line W31 c. When the control value reaches the mitigationvalue W24 c at a time point Q12 c, as shown in a graph of thechange-over valve control signal corresponding to the line W31 c, thecontroller 60 outputs a signal to magnetize the solenoid of thetraveling change-over valve 34 to switch the traveling change-over valve(switching valve) 34 from the first state (first speed) to the secondstate (second speed). In addition, after the time point Q12 c, thecontrol value is restored to the control value W22 before the reduction,as rising along the line W31 b.

Now, focusing on each of mitigation periods Ta, Tb and Tc from the timepoint Q11, at which the control value of the control signal starts to bereduced, to each of the time points Q12 a, Q12 b and Q12 c, at which thereduction of the control value of the control signal stops, i.e., themitigation period Ta, Tb or Tc for which the control value of thecontrol signal is reduced until reaching the corresponding mitigationvalue W24 a, W24 b or W24 c, the controller 60 keeps the first reductionrate of the control value constant. That is, for each of the mitigationperiods Ta, Tb and Tc, the controller 60 keeps the slope of each of thelines W31 a, W31 b and W31 c constant.

The traveling change-over valve 34 is switched between the first stateand the second state at each of the time points Q12 a, Q12 b and Q12 c.Therefore, the controller 60 is configured to change a timing forswitching the traveling change-over valve 34 from the first state to thesecond state in corresponding to the straight-traveling degree SV.

In the second embodiment described above, for each of the mitigationperiods Ta, Tb, and Tc, the control value (opening degree) of thecontrol signal is reduced at a reduction rate kept constant for thecorresponding mitigation period Ta, Tb or Tc from the start to the end.Alternatively, at an intermediate point of the mitigation period, thereduction rate may be changed.

FIG. 8 shows a modified example in which the reduction rate of thecontrol value of the control signal is changed at the intermediate pointin the mitigation period Ta.

When the controller 60 obtains the first speed instruction andcalculates the mitigation value W24 a based on the straight-travelingdegree SV, as shown in FIG. 8 , the controller 60 sets the reductionrate of the control value, in the section (first section) Ta1 defined asa first part of the mitigation period Ta from the start to theintermediate point of the mitigation period Ta to a second reductionrate, and sets the reduction rate of the control value, in the section(section section) Ta2 defined as a second part of the mitigation periodTa from the intermediate point to the end, to a third reduction rate.That is, while the line W31 a indicates variation of the control valuefor the mitigation period Ta, the controller 60 sets the secondreduction rate in the first section Ta1 based on a slope of a line W31 a1 in the line W31 a, and sets the third reduction rate for the secondsection Ta2 in the mitigation period Ta based on a slope of a line W31 a2 in the line W31 a. The controller 60 sets the second reduction rate(slope of line W31 a 1) as a value greater than the third reduction rate(slope of line W31 a 2).

In the above-mentioned modified example, the second and third reductionrates setting based on the line W31 a is described. However, other linesW31 b and 11 c may set the second and third reduction rates in the sameway as the setting with the line W31 a. In this case, the mitigationvalue W24 a should be read as the mitigation value W24 b or W24 c, themitigation period Ta should be read as the mitigation period Tb or Tc,the line W31 a should be read as the line W31 b or W31 c, the line W31 a1 should be read as a line W31 b 1 or W31 c 1, the line W31 a 2 shouldbe read as a line W31 b 2 or W31 c 2, the first section Ta1 should beread as a first section Tb1 or Tc1, and the second section Ta2 should beread as a second section Tb2 or Tc2.

In the second embodiment described above, the traveling operation device54 is hydraulically operated to change pilot pressures acting on thetraveling pumps (first traveling pump 53L and second traveling pump 53R)by means of an operation valve 55, but as shown in FIG. 9 , thetraveling operation device 54 may be an electrically operable device.

As shown in FIG. 9 , the traveling operation device 54 is provided withthe operation member 59 swingable in the lateral direction (machinewidth direction) or in the fore-and-aft direction, and proportionalsolenoid valves serving as the operation valves 55 (including theoperation valves 55A, 55B, 55C, and 55D). The controller 60 is connectedto an operation detection sensor which detects an operation amount andoperational direction of the operation member 59. The controller 60controls the operation valves 55 (including the operation valves 55A,55B, 55C, and 55D) based on the operation amount and operationaldirection detected by the operation detection sensor.

When the operation member 59 is operated to the front (the direction A1,see FIG. 5 ), the controller 60 outputs a control signal to theoperation valves 55A and 55C to swing the swash plates of the firsttraveling pump 53L and the second traveling pump 53R in the directionfor normally rotating the first traveling motor 36L and the secondtraveling motor 36R (for forward traveling).

When the operation member 59 is operated to the rear (the direction A2,see FIG. 5 ), the controller 60 outputs a control signal to theoperation valves 55B and 55D to swing the swash plates of the firsttraveling pump 53L and the second traveling pump 53R in the directionfor reversely rotating the first traveling motor 36L and the secondtraveling motor 36R (for backward traveling).

When the operation member 59 is operated to the right (the direction A3,see FIG. 5 ), the controller 60 outputs a control signal to theoperation valve 55A and the operation valve 55D to swing the swash plateof the first traveling pump 53L in the direction for normally rotatingthe first traveling motor 36L and to swing the swash plate of the secondtraveling pump 53R in the direction for reversely rotating the secondtraveling motor 36R.

When the operation member 59 is operated to the left (the direction A4,see FIG. 5 ), the controller 60 outputs a control signal to theoperation valve 55B and the operation valve 55C to swing the swash plateof the first traveling pump 53L in the direction for reversely rotatingthe first traveling motor 36L and to swing the swash plate of the secondtraveling pump 53R in the direction for normally rotating the secondtraveling motor 36R.

The working machine 1 includes the prime mover 32, the traveling pumps53L and 53R driven by power of the prime mover 32 so as to deliveroperation fluid, the traveling motors 36L and 36R configured to berotated by the operation fluid delivered from the traveling pumps 53Land 53R, the traveling motors 36L and 36R having a rotation speedshiftable between a first speed and a second speed that is higher thanthe first speed, the machine body 2 provided thereon with the primemover 32, the traveling pumps 53L and 53R and the traveling motors 36Land 36R, the traveling change-over valve 34 shiftable between the firststate where the rotation speeds of the traveling motors 36L and 36R areset to the first speed and the second state where the rotation speeds ofthe traveling motors 36L and 36R are set to the second speed, thetraveling operation device 54 including the operation valve 55configured to change a pressure of operation fluid to be applied to thetraveling pumps 53L and 53R according to operation of the operationmember 59, the actuation valve 69 provided upstream or downstream of theoperation valve 55 to be fluidly connected to the operation valve 55 sothat the actuation valve 69 is operable to control a flow of operationfluid to the operation valve 55, and the controller 60 configured orprogrammed to perform a shock-mitigation control for reducing an openingdegree of the actuation valve 69 by outputting a control signal to theactuation valve 69 when the traveling change-over valve 34 is shiftedfrom the first state to the second state. The controller 60 isconfigured or programmed to determine the reduction amount of theopening degree of the actuation valve 69 reduced by the shock-mitigationcontrol, based on a straight-traveling degree of the machine body 2.

According to this configuration, since the straight-traveling degree ofthe machine body 2 is determined based on the direction in which adriver (operator) operates the operation member 59 of the travelingoperation device 54, the opening degree of the actuation valve 69 isreduced according to the straight-traveling degree of the machine body2, that is, according to the operation by the driver (operator), so thatthe acceleration (switching from the first state to the second state)can be performed while reducing the shift shock as intended. In otherwords, when a pressure acting on the operation valve 55 is changed bydecreasing the opening degree of the actuation value 69 in order toreduce the shift shock, it is possible to realize a balance between thepressure of hydraulic fluid output from the operation valve 55 to thetraveling pumps 53L and 53R and the straight traveling degree of themachine body 2 actually traveling (operation by the driver (operator)).Accordingly, the shift shock can be reduced while suppressing therattling of the working machine 1 due to the shock-mitigation control.

The actuation valve 69 is configured so that the opening degree thereofbecomes larger as the control value corresponding to the control signalbecomes larger, and the opening degree thereof becomes smaller as thecontrol value becomes smaller, and the controller 60 is configured orprogramed to determine the reduction amount ΔF11 of the control value asthe reduction amount ΔF11 of the opening degree of the actuation valve69 based on the straight-traveling degree of the machine body 2, and tocalculate the mitigation value W24 a, W24 b or W24 c for theshock-mitigation control based on the reduction amount ΔF11.

According to this configuration, the mitigation value W24 a, W24 b orW24 c of the control value of the control signal output to the actuationvalve 69 can be set according to the straight-traveling degree of themachine body 2, so that the shift shock can be reduced more smoothly.

When the period for the shock mitigation control until the control valuereduced by the shock mitigation control reaches the mitigation value isdefined as the mitigation period Ta, Tb or Tc, the controller 60 isconfigured or programmed to reduce the control value at the firstreduction rate W24 a, W24 b or W24 c kept constant for the mitigationperiod from the start point thereof to the end point thereof.

According to this configuration, the pressure of hydraulic fluid actingon the traveling pumps 53L and 53R can be lowered as smoothly aspossible, so that the shift shock can be reduced smoothly withoutdiscomfort.

When the period for the shock mitigation control until the control valuereduced by the shock mitigation control reaches the mitigation value isdefined as the mitigation period Ta, Tb or Tc, the controller 60 isconfigured or programmed to reduce the control value at the secondreduction rate for the first part of the mitigation period Ta, Tb or Tcfrom the start point thereof to the intermediate point thereof, and toreduce the control value at the third reduction rate for the second partof the mitigation period Ta, Tb or Tc from the intermediate pointthereof to the end point thereof, the second reduction rate beinggreater than the third reduction rate.

According to this configuration, the responsiveness of the actuationvalve 69 can be improved in shock-mitigation control.

The controller 60 is configured or programmed to change the timing forshifting the traveling change-over valve 34 from the first state to thesecond state in correspondence to modified example in thestraight-traveling degree.

According to this configuration, the timing of acceleration can bechanged suitably for the straight traveling of the machine body 2 andfor the pivot turn or the like of the machine body 2, so thatworkability can be further improved.

The controller 60 is configured or programmed to determine the reductionamount ΔF11 based on the straight-traveling degree in such a way thatthe reduction amount ΔF11 becomes larger as the straight-travelingdegree becomes larger, and the reduction amount ΔF11 becomes smaller asthe straight-traveling degree becomes smaller.

According to this configuration, for example, when the machine body 2 istraveling straight, the shift shock in the acceleration can be furtherreduced in the straight traveling by increasing the reduction amountΔF11, and when the machine body 2 is changing its traveling from thestraight traveling to the pivot turn or when the machine body 2 ispivotally turning, the reduction amount ΔF11 is reduced so that theshift shock can be reduced in a stable manner while maintaining, in thepumps 53L and 53R, a pressure difference (differential pressure) betweenthe hydraulic fluids acting on the traveling motors 36L and 36R for thenormally-rotating direction or the reversely rotating direction.

The working machine 1 further includes the first traveling device 5Lprovided on a left portion of the machine body 2, and the secondtraveling device 5R provided on a right portion of the machine body 2.The traveling motor 36L serving as a first traveling motor configured tooutput a traveling power to the first traveling device 5L and thetraveling motor 36R serving as a second traveling motor configured tooutput a traveling power to the second traveling device 5R are eachprovided as the traveling motor. The traveling pumps 53L and 53R areconfigured to rotate the first traveling motor and the second travelingmotor. The traveling change-over valve 34 is configured to shift therotation speed of the first and second traveling motors between thefirst speed and the second speed.

This makes it possible to mitigate the shift shock more smoothly in theworking machine 1 having the first traveling device 5L on the leftportion of the machine body 2 and the second traveling device 5R on theright portion of the machine body 2.

In the above description, the embodiment of the present invention hasbeen explained. However, all the features of the embodiment disclosed inthis application should be considered just as examples, and theembodiment does not restrict the present invention accordingly. A scopeof the present invention is shown not in the above-described embodimentbut in claims, and is intended to include all modifications within andequivalent to a scope of the claims.

What is claimed is:
 1. A working machine comprising: a prime mover; a traveling pump driven by power of the prime mover so as to deliver operation fluid; a traveling motor configured to be rotated by the operation fluid delivered from the traveling pump, traveling motor having a rotation speed shiftable between a first speed and a second speed that is higher than the first speed; a machine body provided thereon with the prime mover, the traveling pump and the traveling motor; a traveling change-over valve shiftable between a first state where the rotation speed of the traveling motor is set to the first speed and a second state where the rotation speed of the traveling motor is set to the second speed; a traveling operation device including an operation valve configured to change a pressure of operation fluid to be applied to the traveling pump according to operation of an operation member; an actuation valve provided upstream or downstream of the operation valve to be fluidly connected to the operation valve so that the actuation valve is operable to control a flow of operation fluid to the operation valve; and a controller configured or programmed to perform a shock-mitigation control for reducing an opening degree of the actuation valve by outputting a control signal to the actuation valve when the traveling change-over valve is shifted from the first state to the second state, wherein the controller is configured or programmed to determine a reduction amount of the opening degree of the actuation valve reduced by the shock-mitigation control, based on a straight-traveling degree of the machine body.
 2. The working machine according to claim 1, wherein the actuation valve is configured so that the opening degree thereof becomes larger as a control value corresponding to the control signal becomes larger, and the opening degree thereof becomes smaller as the control value becomes smaller, and the controller is configured or programed to determine a reduction amount of the control value as the reduction amount of the opening degree of the actuation valve based on the straight-traveling degree of the machine body, and to calculate a mitigation value for the shock-mitigation control based on the reduction amount.
 3. The working machine according to claim 2, wherein when a period for the shock mitigation control until the control value reduced by the shock mitigation control reaches the mitigation value is defined as a mitigation period, the controller is configured or programmed to reduce the control value at a first reduction rate kept constant for the mitigation period from a start point thereof to an end point thereof.
 4. The working machine according to claim 3, wherein the controller is configured or programmed to change a timing for shifting the traveling change-over valve from the first state to the second state in correspondence to the straight-traveling degree.
 5. The working machine according to claim 4, further comprising: a first traveling device provided on a left portion of the machine body; and a second traveling device provided on a right portion of the machine body, wherein a first traveling motor configured to output a traveling power to the first traveling device and a second traveling motor configured to output a traveling power to the second traveling device are each provided as the traveling motor, the traveling pump is configured to rotate the first traveling motor and the second traveling motor, and the traveling change-over valve is configured to shift the rotation speed of the first and second traveling motors between the first speed and the second speed.
 6. The working machine according to claim 3, wherein the controller is configured or programmed to determine the reduction amount based on the straight-traveling degree in such a way that the reduction amount becomes larger as the straight-traveling degree becomes larger, and the reduction amount becomes smaller as the straight-traveling degree becomes smaller.
 7. The working machine according to claim 3, further comprising: a first traveling device provided on a left portion of the machine body; and a second traveling device provided on a right portion of the machine body, wherein a first traveling motor configured to output a traveling power to the first traveling device and a second traveling motor configured to output a traveling power to the second traveling device are each provided as the traveling motor, the traveling pump is configured to rotate the first traveling motor and the second traveling motor, and the traveling change-over valve is configured to shift the rotation speed of the first and second traveling motors between the first speed and the second speed.
 8. The working machine according to claim 2, wherein when a period for the shock mitigation control until the control value reduced by the shock mitigation control reaches the mitigation value is defined as a mitigation period, the controller is configured or programmed to reduce the control value at a second reduction rate for a first part of the mitigation period from a start point thereof to an intermediate point thereof, and to reduce the control value at a third reduction rate for a second part of the mitigation period from the intermediate point thereof to an end point thereof, the second reduction rate being greater than the third reduction rate.
 9. The working machine according to claim 8, wherein the controller is configured or programmed to change a timing for shifting the traveling change-over valve from the first state to the second state in correspondence to the straight-traveling degree.
 10. The working machine according to claim 8, wherein the controller is configured or programmed to determine the reduction amount based on the straight-traveling degree in such a way that the reduction amount becomes larger as the straight-traveling degree becomes larger, and the reduction amount becomes smaller as the straight-traveling degree becomes smaller.
 11. The working machine according to claim 8, further comprising: a first traveling device provided on a left portion of the machine body; and a second traveling device provided on a right portion of the machine body, wherein a first traveling motor configured to output a traveling power to the first traveling device and a second traveling motor configured to output a traveling power to the second traveling device are each provided as the traveling motor, the traveling pump is configured to rotate the first traveling motor and the second traveling motor, and the traveling change-over valve is configured to shift the rotation speed of the first and second traveling motors between the first speed and the second speed.
 12. The working machine according to claim 2, wherein the controller is configured or programmed to change a timing for shifting the traveling change-over valve from the first state to the second state in correspondence to the straight-traveling degree.
 13. The working machine according to claim 12, further comprising: a first traveling device provided on a left portion of the machine body; and a second traveling device provided on a right portion of the machine body, wherein a first traveling motor configured to output a traveling power to the first traveling device and a second traveling motor configured to output a traveling power to the second traveling device are each provided as the traveling motor, the traveling pump is configured to rotate the first traveling motor and the second traveling motor, and the traveling change-over valve is configured to shift the rotation speed of the first and second traveling motors between the first speed and the second speed.
 14. The working machine according to claim 2, wherein the controller is configured or programmed to determine the reduction amount based on the straight-traveling degree in such a way that the reduction amount becomes larger as the straight-traveling degree becomes larger, and the reduction amount becomes smaller as the straight-traveling degree becomes smaller.
 15. The working machine according to claim 2, further comprising: a first traveling device provided on a left portion of the machine body; and a second traveling device provided on a right portion of the machine body, wherein a first traveling motor configured to output a traveling power to the first traveling device and a second traveling motor configured to output a traveling power to the second traveling device are each provided as the traveling motor, the traveling pump is configured to rotate the first traveling motor and the second traveling motor, and the traveling change-over valve is configured to shift the rotation speed of the first and second traveling motors between the first speed and the second speed.
 16. The working machine according to claim 1, wherein the controller is configured or programmed to change a timing for shifting the traveling change-over valve from the first state to the second state in correspondence to the straight-traveling degree.
 17. The working machine according to claim 16, further comprising: a first traveling device provided on a left portion of the machine body; and a second traveling device provided on a right portion of the machine body, wherein a first traveling motor configured to output a traveling power to the first traveling device and a second traveling motor configured to output a traveling power to the second traveling device are each provided as the traveling motor, the traveling pump is configured to rotate the first traveling motor and the second traveling motor, and the traveling change-over valve is configured to shift the rotation speed of the first and second traveling motors between the first speed and the second speed.
 18. The working machine according to claim 1, wherein the controller is configured or programmed to determine the reduction amount based on the straight-traveling degree in such a way that the reduction amount becomes larger as the straight-traveling degree becomes larger, and the reduction amount becomes smaller as the straight-traveling degree becomes smaller.
 19. The working machine according to claim 18, further comprising: a first traveling device provided on a left portion of the machine body; and a second traveling device provided on a right portion of the machine body, wherein a first traveling motor configured to output a traveling power to the first traveling device and a second traveling motor configured to output a traveling power to the second traveling device are each provided as the traveling motor, the traveling pump is configured to rotate the first traveling motor and the second traveling motor, and the traveling change-over valve is configured to shift the rotation speed of the first and second traveling motors between the first speed and the second speed.
 20. The working machine according to claim 1, further comprising: a first traveling device provided on a left portion of the machine body; and a second traveling device provided on a right portion of the machine body, wherein a first traveling motor configured to output a traveling power to the first traveling device and a second traveling motor configured to output a traveling power to the second traveling device are each provided as the traveling motor, the traveling pump is configured to rotate the first traveling motor and the second traveling motor, and the traveling change-over valve is configured to shift the rotation speed of the first and second traveling motors between the first speed and the second speed. 